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What is the difference between a turbofan and a turboprop engine?
I'm not an aircraft expert and I just realized that there are two different terms - turbofan and turboprop. I always had them combined in my head as a term for a large jet engine (like you would see on a 747). Apparently this is not the case though. So what is the difference between them? Are there other types of "turbo" engines?

fooot's answer is excellent. Large modern commercial airliners use turbofan engines; you still see turboprops in use in smaller regional passenger planes. Most modern military aircraft are turbofan-driven, though types that prioritize endurance over speed will use turboprops.

Possible duplicate of What are the differences between fuel types (comparing with vehicles)?

@RANSARA009 These questions are not duplicated at all....

Both engines use a turbine for power. This is where the "turbo" part of the name comes from. In a turbine engine, air is compressed and then fuel is ignited in this compressed air. The energy produced by the ignition turns the turbine. The turbine is then able to drive both the compressor at the front of the engine and also some useful load. In airplanes, it produces thrust.
The first jet engine was a turbojet. This is a simple turbine engine that produces all of its thrust from the exhaust from the turbine section. However, because all of the air is passing through the whole turbine, all of it must burn fuel. This means it is inefficient, and the solution is the turbofan.

In a turbofan, the turbine primarily drives a fan at the front of the engine. Most engines drive the fan directly from the turbine. There are usually at least two separate shafts to allow the fan to spin slower than the inner core of the engine. The fan is surrounded by a cowl which guides the air to and from the fan. Part of the air enters the turbine section of the engine, and the rest is bypassed around the engine. In high-bypass engines, most of the air only goes through the fan and bypasses the rest of the engine and providing most of the thrust.

In a turboprop, the turbine primarily drives a propeller at the front of the engine. There is no cowl around the prop. Some air enters the turbine, the rest does not. The propeller is geared to allow it to spin slower than the turbine. Although this diagram shows only a single shaft, many turboprops have two, with a high pressure shaft driving the compressor and a low pressure shaft driving the propeller. Some engines such as the popular PT6 also reverse the flow direction multiple times.

Turboprops are more efficient at lower speeds since the prop can move much more air with a smaller turbine than the fan on a turbofan engine. The cowl around the turbofan's large fan allows it to perform better than an open propeller at high speeds, but limits the practical size of the fan.
At supersonic speeds, turbojets have more of a performance benefit. They develop all of their thrust from the high velocity turbine exhaust, while turbofans supplement that with the lower velocity air from the fan. Since the air from the fan is also not compressed nearly as much as the core turbine flow, it is also harder to prevent the flow from going supersonic and causing losses.
The Concorde used turbojets because it was designed to cruise for long periods at supersonic speeds. Modern fighter jet engines are turbofans, which provide a compromise between efficiency and speed.
There are other benefits and drawbacks between turbojets, turbofans, and turboprops, but I think they are beyond the scope of this question.
Work has been done on creating a "propfan" engine, in an attempt to get the efficiency of a turboprop and the speed of a turbofan. They have yet to come up with a viable design.

Elsewhere in aviation, turbine engines are used in

helicopters, as a turboshaft engine driving the rotors instead of a propeller, and with a freewheeling clutch to enable autorotations
APUs in jet aircraft and large turboprop aircraft

Turbines also find use outside of aviation in power plants (to generate electricity), and even vehicles (like the Abrams tank).
Turbocharged piston engines use a turbine much differently from the examples above. Instead of being the primary power source, the turbine only assists the piston engine. A turbocharger uses a turbine to compress air sent to the engine intake. The increased compression helps the engine generate more power. The turbine of a turbocharger is driven by engine exhaust gasses, and a supercharger is similar but is directly powered by the engine. See the Wikipedia page for more detail.

Great answer. Now, are turbojets better than turbofans for high altitude supersonic flight? I see most 1960s fighter jets and the Concorde used turbojet engines...

@fooot Great, informative answer.  You may want to mention turbocharged piston engines.  That was a confusion for me when I first began researching the different types of airplane engines.

Minor correction to "turbojet ... that produces all of its thrust from the turbine": thrust is never produced in the turbine. It takes thrust away from the exhaust and use it to drive the compressor (and, where present, fans or props). In a turbojet all the thrust there is is that which is not taken out of the exhaust gases by the turbine.

I would also add a correction to the sentence "turbojet ... that produces all of its thrust from the turbine". The thrust is also generated by the Air Intake sucking air into the engine.

And I would also add that the turbine engine used in a helicopter is called "Turbo shaft".

A400m and C130 are good examples of turboprop and also prove why mission planes require turboprop more than turbofan...

Why is the exhaust air ducted down in the turbo prop example? Wouldn't it make sense to have it shoot straight through?

@RoboKaren: Don't think of it as ducted down. Rather, think of it as being fed to an exhaust. Turboprops, like piston engines, are normally installed in front of the plane. Thus you need to deflect the exhaust somewhere to prevent it from blasting the cockpit and passenger compartment. If the aircraft doesn't have this restirction (twin engine mounted on the wings for example) then it would indeed make sense to just vent the jet straight through. Indeed some real-world designs do exactly this.

@RoboKaren: The PT-6A (a common turboprop engine) is "backwards."  Air enters at the back, works its way forwards and exits out exhausts on the the sides.  The main turbine is on the front, tied to a gearbox, and then the propeller. The nature of turbojets and turbofans is that the exhaust gasses have significant velocity.  With a turboprop, there's relatively little velocity in the exhaust.  Most of the energy in the exhaust has been captured by the main turbine and turned into rotational power.

Yes the turboprop depicted above is a Garrett-style turboprop with a gearbox. They are noticeably louder then the freewheeling Pratt&Whitneys

Actually, in the PT-6A the air enters from the front, is ducted towards the back, goes through an inertial separator, and is fed into the back of the engine. Small but important difference.

It should be mentioned that like turbofans, turboprops also usually have two turbine sections, the high-pressure section driving the compressor and the low-pressure section driving the propeller (or shaft). The image does not reflect this. It also explains why some turboprops, like the PT-6A, has the air flow back-to-front—it avoids the concentric shaft.

Further to above answer..,
Check out this link to see inside of an engine http://animagraffs.com/inside-a-jet-engine/
Turbofan is the most efficient engine and most of the thrust is produced from the bypass air. The efficiency of the engine mainly depends on the engine bypass ratio. Turbojet is mainly used on the fighter or acrobatic aircraft. These aircraft’s thrust is further increased by the “after burner” installed at the nozzle of the engine.
Turbo propellers are efficient on the short overhaul operation and the a/c with turbo props requires only shorter runway to achieve the required lift compared to other types.
Turbo shaft, is the engine mainly used on the helicopter and the energy comes out of the turbine is used by the rotor to achieve the required rpm.


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What are these 4 red spots on the A-10?
What are these 4 spots on the wings and fuselage of the A-10? I can't seem to find a closer shot of their wings but each spot looks like a small target with a dot and an outer circle, mostly in red, but some are in black.

They put those there for the enemy to target and become disheartened after scoring direct hits on all four of them and she still flies.

I am fairly sure that these are the fuel hatches. They are placed right over the wing and main fuselage fuel tanks. (The hatches are where the fuel is put into the craft, not drawn out for sampling). During ground fueling, the hatches are used to refill the tanks.

Yes, @Nicholas is correct, they are fuel tank caps. Normal ground refuelling is in the left wing pylon but if you don't have the proper equipment you can gravity fuel just like any other fuel tank. Two wing tanks, two fuselage tanks.

See #84 on this drawing

Yeah: I immediately thought 'fuel caps' and it appears to be the correct answer. (Never seen them called "fuel hatches" but I suppose that has to do with what form of English one is using?)

Nicholas got the geist of this one.  The red circles are marking on the four gravity fill fuel tank ports.  The A-10 also uses a pressure fueling port is located on the port landing gear fairing behind an enclosure for most refueling since it is easier and faster.

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How was flutter mitigated in jets such as the F-15 and F-16?
As I understand it flutter was a serious problem for early jets, like the XF-104. Flutter can affect any of the extended members of an aircraft during high-speed flight, including the wings and the vertical stabilizer. The problem seems to have solved, however, by the time that the F-16 and F-15 were designed.
How is flutter mitigated in the F-15 and F-16?

Short answer: By not  flying faster than the 104 did and adhering to the lessons learned.
Flutter first started to cause crashes in WW I when improved engine power and aerodynamics made a substantial rise in flight speed possible. Every time technological advance allowed higher speeds, flutter became an issue which was then solved both by trial and error and by analytics.
From NASA TM 4720:

The first recorded flutter incident was on a Handley Page O/400 twin
  engine biplane bomber in 1916. The flutter mechanism consisted of a
  coupling of the fuselage torsion mode with an antisymmetric elevator
  rotation mode.

This was easily fixed by removing one degree of freedom for the elevator: Initially, both sides could be moved independently, and by connecting them with a torsion tube the flutter mode could be avoided.
From control surface flutter in WW I, which was generally solved by the practice of balancing all control surfaces (something the early aircraft designers were oblivious of), flutter problems moved to primary surfaces from 1925 on. Again NACA TM 4720:

Air racers experienced many incidents of flutter from the mid-1920's
  until the mid-1930's as attempts were made to break speed records

The first analytical testing was performed by von Schlippe in 1935 in Germany. He vibrated the aircraft at increasing speeds and measured the resulting amplitude. An increase in amplitude indicated a reduction in damping and imminent danger of flutter.

Typical amplitude over speed plot (from NACA TM 806, which is a direct translation of von Schlippe's work which was first published in Luftfahrtforschung Vol. 13 No. 2, February 1936)
With jets and their higher speeds, new types of flutter occurred. Again NACA TM 4720:

Supersonic speeds also produced a new type of flutter known as panel
  flutter. Panel flutter involves constant amplitude standing or
  traveling waves in aircraft skin coverings. This type of instability
  could lead to abrupt fatigue failure, so the avoidance of panel
  flutter is important.

Since maybe fifty years every new design must go through a static vibration test to check which elastic eigenfrequencies exist. Analytical methods are used to predict the (speed dependent) aerodynamic frequencies and the structure must be modified to remove any possibility of resonance. Next, the airframe is tested in flight at ever increasing speeds and with exciters at the tips of all surfaces (those can be like the vibration motor in your smartphone, only bigger, or, in the simplest of cases, the pilot moves the control surfaces). At increasing speeds, the exciters are run through a frequency sweep (typically from 5 to 60 Hz) and the resulting amplitude is measured by strain gages or accelerometers.
Below is a diagram (taken from NACA TM 4720) depicting the flutter test of the F-14. The F-14 and F-16 went through the same procedure to make sure flutter is no issue in operation. Modern fly-by-wire designs can even use their control surfaces for active flutter suppression.

The XF-104 is a prototype aircraft, which lead to the F-104 starfighter. Usually, during the development of an aircraft, flutter testing is carried out and corrective actions are taken. The Wikipedia article you liked has a point:

Production aircraft would also feature a redesigned fin structure using stainless steel spars to eliminate the flutter problem.

In a similar manner, during flight testing of F-15 and F-16 (or any other aircraft, for that matter), various modes of flutter are identified and corrective actions are taken. For example, in case of F-15, flutter in the empennage was observed during tests, which was corrected by redesigning the stabilator (by introducing a 'snag').

F-15 stabilator design changes due to flutter; image from airandspacemuseum.org
Some modifications were also carried out on the vertical tail (by increasing stiffness). So basically, the answer is the flutter modes are (hopefully) identified during the test program of the aircraft and appropriate corrective measures taken.

After the flutter test and  identification of the aircraft envelope prone to create divergent fluttering, then stiffening of the aircraft wings and rudders, stabilators, etc... the another way was to prohibited maneuvers in that region of the flight (rolling under the hard G's and supersonic speeds). F 15 rudders are not that tall just for that reason

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Under what conditions would a helicopter be allowed to fly at low altitude over populated areas?
Yesterday, the Department of Energy was flying a military helicopter at low altitudes over Boston and the surrounding area as part of an experiment of some type to measure radiation.
I saw one of these aircraft when I driving through Watertown and in fact it passed right over me. It was operating at a very dangerous altitude. Any kind of engine failure would have created a multiple fatality situation. Normally, aircraft do not fly at high speed below 500 feet and when helicopters descend below 500 feet they normally do so cautiously and in an area they know to be safe. Having a large helicopter flying at high speed over a dense city at 100 feet AGL is very dangerous and against FAA regulations.
The type of helicopter was not the civvy type pictured in the news story, it was a Chinook. For NASA to be circulating a photo with a little civvy helicopter and then actually flying around a big Chinook I thought was pretty deceptive on their part.
I have not filed a pilot complaint about this because I assume some kind of special "permission" or exception was created to allow for this dangerous activity. Does anybody know the legal authority involved here and conditions?
BTW, I did not check the NOTAMs and did not get any NOTAMs in my mailbox on this, so if somebody can link the NOTAM that would be useful.

Update: this may have been a medevac flight, with emphasis on the word "may". Apparently, in some cases military helicopters are used for medevac flights to Burlington. However, if it was, the pilot was hot dogging and off the normal routes, both of which are serious regulation violations, but nevertheless I am told some military pilots disregard the rules.

Any kind of engine failure would have created a multiple fatality situation. I believe that this is the flawed premise that negates the entire question.  For a Chinook, an engine failure in high speed forward flight is fully controllable - indeed, the type will not gain certification if it is not.  Forgive me, but the entire post reads more like a rant than a serious enquiry.

CFR 91.119(d) "A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA" Also see Boston Helicopter Routes (Click on the Helicopters tab). You are also 2 days early, and they say they are flying a grid pattern, probably approved by the FAA.

I note that the announcement (dated April 11) explicitly talks about a "twin-engine Bell 412" operating "April 12 through April 15". How sure are you that the different helicopter you saw on April 10 is actually anything to do with the NNSA program? The logical assumption would be that they're unconnected...

It was a safety yellow Chinook 147, not a Bell, and it was AGL 100. If a Chinook has an engine failure it will rapidly lose altitude which is fatality-inducing situation if you are flying at 100 feet AGL.

@RonBeyer That rule has nothing to do with flying 100 feet over Watertown square. There is no such "prescribed route". As far as "permission", I know that and I stated it in my question. I want to know what that "permission" is that is allowing a highly dangerous operation like this to be conducted.

It should not rapidly lose altitude, what do you base that on? The "permission" that is allowing this to be conducted is national security.

I have been in a Chinook, at about 50 feet, and around 70kts, with a simulated engine failure. Have you?  It does not lose height rapidly.  The recovery in this case (flying through a fire break in a forest) was to climb to a more normal altitude so that an auto was possible if the second engine quits.

I have not filed a pilot complaint about this because I assume some kind of special "permission" or exception was created to allow for this dangerous activity. you clearly have a bee in your bonnet about this.  Why not report it? https://www.faa.gov/contact/

Was it NNSA or NASA?

Are you implying that this was over a congested area?

How do you know it was 100ft? Chinooks are big, you could easily have misjudged its altitude.

NASA doesn't own a CH-47 according to this:  https://airbornescience.nasa.gov/aircraft  And a CH-47 is 98 ft long (with rotors) so the claim that it was about an aircraft length above the ground also seems unlikely (although not impossible)

It's important to make the general note that, although it initially seems illogical, most helicopters have a better chance of recovering from engine failure at 100 knots and 100 ft AGL than they do at 20 knots and 250 feet.    So the traveling at a high speed instead of hovering probably improved the chance that the helicopter you saw could recover from engine failure.  See https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velocity_diagram for an explanation.

Was it the DOE, the military or NASA? Make your mind up. Those are three completely separate organizations.

By your description a "military" Chinook-type helicopter was flying at high speed over a congested area at about 100 ft AGL.
The answer to your title question—Under what conditions would a helicopter be allowed to fly at low altitude over populated areas?—is twofold.

Firstly, under FAA regulations (more on this in a moment), helicopters are not generally subject to any specific altitude restrictions. See 14 CFR 91.119 Minimum safe altitudes: General.

Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA

The FAA comments on this regulation:

Helicopter operations may be conducted below the minimum altitudes set for fixed-wing aircraft. The reason? The helicopter's unique operating characteristics, the most important of which is its ability to execute pinpoint emergency landings during power failure. Further, the helicopter's
  increased use by law enforcement and emergency medical service agencies requires added
  flexibility in the application of many FAA provisions.

Therefore, a helicopter may be operated at an altitude which complies with §91.119 (a) and (d), despite perhaps appearing to be otherwise to the unknowing observer.
Secondly, military aircraft operations are generally not subject to the same FAA regulations that civilian aircraft operations are. Therefore a military helicopter operation, such as that which you describe, might well be operated under military aviation rules in disregard to rules that would otherwise govern civilian helicopter operations. See this answer regarding the FAA's jurisdiction—or lack thereof—over military aircraft operations.

Taken together, we see that the military helicopter operation that you describe might well have been operated in compliance with 14 CFR 91.119—and according to Simon's comment above, it probably was; we also see, however, that the military will operate according to it's own rules. The answer to the question in the body of your text, Does anybody know the legal authority involved here and conditions?, can best be answered: "by the authority of the US Government, under whatever conditions they deemed appropriate".

I have no idea about the specific flight you saw, but the general answer to your question about operating legally below the minimum safe altitudes in 14 CFR 91.119 is that the FAA can grant a waiver allowing lower flight. The waiver process says that to get a waiver to 91.119(b) (flight over congested areas) there must be a public interest reason but aircraft can't be lower than 500ft unless lives are at stake:

When issuing the waiver, the altitude requested should be an absolute
  minimum, but may not be less than 500 feet from persons or property,
  unless necessary to safeguard human life

There isn't enough information about the helicopter you saw to say what was going on but assuming that it really was a Chinook at at 100' AGL then it was too low even for a waiver, according to the FAA's process. But the most obvious explanation is simply that it was taking off; helicopters take off in a range of attitudes and airspeeds and some of those takeoffs are probably relatively low and fast. It might have been a medevac flight (they're often brightly painted) or perhaps something to do with logistics or construction; if you got the N number then you could look it up.

This was my general impression. Going below 500 feet is a no-no, so I am trying to figure out how it is, that this is happening. The chopper I saw was no medevac flight.

@TylerDurden Why do you say that going below 500 feet is a no-no? On what basis? We are talking about helicopter operations here, aren't we? And military helicopter operations, what is more. I am curious to know what I am missing regarding why low level flight would be prohibited.

The quick, short answer -  It depends.
Remember, there are two distinct sets of rules involved here.

The FARs, they apply to the vast majority of aircraft (Fixed wing and Rotary wing).
Public use aircraft - military, local government aircraft and pilots.
These aircraft and pilots are not necessarily operating under the control/regulation of the FAA.  Just because the City, County or State entities require the pilots to be licensed doesn't mean they must do this.


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Why aren’t diesel piston engines used on aircraft like the Saab 340 or L-410?
In contrast to this question (link), I would like to know the opposite - why diesel piston engines aren't used on aircraft with comparable role and configuration to Saab 340, L-410, ATR-42?
There's a very similar question here, but it was mainly focused on small GA aircraft and private owners, while I'm more interested in commuter type piston airplanes.
Here's my thoughts so far after some reading (please note, some of them are just my assumptions and might be wrong).
Pros of diesel engine:

Engine itself is significantly cheaper
Cheaper to maintain
More economical according to this discussion (link)

Cons of diesel engine (vs turbine):

Likely they are heavier and more bulkier than turbine of similar
power output (link to similar discussion)
Diesel engine pylon is less aerodynamic (more drag) due to presence
of necessary air inlets for engine cooling
Possibly altitude (and thus performance) limited. I'm not really sure
about this point though. It's a well-known fact that commuter
turboprops can fly at 20-25k feet. I don't have any data for diesel
piston engines. I can only assume that diesel engine with a
turbo-charger might actually reach similar altitudes and performance.
Require more frequent maintenance

Perhaps this is worth asking as a separate question.
How heavy and big would be diesel piston equivalent of PW120 engine? (some ideas can be found in this discussion)
PW120 EASA data for weight, dimension and thrust (link):

Length 2130 mm
Width 635 mm
Dry weight 417.3 kg
Take off shaft power 1342 kW
Maximum Continuous shaft power 1268 kW

"Engine itself is significantly cheaper, Cheaper to maintain" Really?

There's several questionable assumptions here @ElectricPilot, the engines are not cheaper, nor cheaper to maintain or more economical. Your discussion links go to wikipedia pages. What are you basing these assumptions on? Don't you think if these were correct we'd be using 3000hp piston engines instead of turbines?

@GdD, links have been corrected now, I must have edited them in the wrong way.

OT: check this story and history of a WW2 plane from the German airforce powered by diesel piston engines:
https://www.historynet.com/luftwaffes-high-flying-diesel.htm
In short: they solved the weight issue by introducing a design change that would be a nightmare to mantain for civil scope (especially targeting emissions requirments): two-stroke engine, piston against piston.in single bore.

There are many reasons but Ill address your points directly first

Engine itself is significantly cheaper

This is only true for non aviation engines. Even if you were to obtain a type certification for an automotive engine the cost of that would reflect a huge increase in price. This has been tried before and proved so costly the engine maker ended up dropping the FAA type cert due to costs.

Cheaper to maintain

Again this is only true for auto or industrial application engines. Any part carrying approval down to the little snap rings that hold things on has a large price mark up over their non aviation counterpart (even if the parts are more or less identical). On the whole piston engines tend to also have a shorter TBO than turboshaft engines.

Likely they are heavier and more bulkier than turbine of similar power
output

This is perhaps one of the most important cons. In aviation weight is the name of the game. Airlines make money not only on passengers but the carriage of freight in empty baggage space cutting into that cuts into profits.

Piston engines once did rule the skies in that size class, the venerable DC-3 is about as big as what you mention and has long since been phased out of routine use.
For the altitude space in the FL200-FL250 range you also need pressurization which is much harder to drive from a piston engine. A turbine allows you to pull bleed air for pressurization while a piston engine requires a whole separate unit driven by the engine somehow. This adds complexity and again, weight. Over the years there have been some pressurized piston planes but they have fallen out of popularity and use.

In addition to the extra engine weight cutting into how many passengers or how much freight could be carried, it could also reduce how much fuel is carried which would affect the aircraft's range (I think this is actually a more likely scenario).

There are no spark plugs in a diesel (there are preheat plugs, though).

@JanHudec thanks for noting! i updated with a more generic part

Bear with me 1 minute with a brief side note. Someone said that my previous post did not answer the question of Diesel engines vs Turboprops.  And while I was writing this to clarify my other post was deleted so this one has to stand alone. Okay now let me answer the question directly so that nobody has to use the info to make their own comparison.
The Diamond DA-62 is a twin engine with each of the 2.0L turbocharged 4 stroke inline 4 Diesel piston engines putting out 180HP (360 combined) maximum and 378 lbs/ft (756 combined) of torque. 3887 engine max rpm, Equipped with a reduction box of 1:1.69 gives a maximum prop speed of 2300 rpm. Rated at 16.6 mpg but the Austrian data doesn't specify weather that's US gallon or the all but abolished Imperial gallon. So I have fuel burn rates here. Each engine is said to burn 9.25 US gallon/hr @ 100% load on the ground, 12.83 US gallons per hour @ 60% load and 12,000 feet operation,  7.4 US gallon @ loiter.  The plane is 5 - 7 passenger with 1 pilot.  Maximum payload capacity including fuel 1600 lbs.  And yes this information is required to answer the question poised. Each engine weighs 410 lbs dry. The manufacturer has listed 1800 hrs average time to overhaul. Manufacturer reccomends Diesel and/or Kerosene (Kerosene is Jet-A. everyone knows they are the same basic fuel) These engines are adapted from Mercedes automotive diesel road engines. Cheap cheap cheap. Solid proven engine design cars have been known to achieve 1,000,000 miles with the original engine.
A similar size turboprop plane is a bit of an oddity but I did locate specifications on a Cessna Denali which is said to have a full of fuel payload of 1100 lbs. It burns 60 GPH but does not specify at what specific engine load nor altitude.  It is a single turboprop with 1240 horsepower, weighing about 600 lbs dry. No specs as to the prop shaft speed or torque are provided but this is not a diesel engine so one can safely assume it will spin faster and provide lower torque output. The manufacturer lists 4000-6000 mean hrs between overhaul. They do advertise 20% less fuel burned than the same size competitors engine, I take this to mean another brand of Turboprop engine. Rated for use with jet-a type fuel. These engines are newly developed and very expensive.
The Cessna has a slightly higher maximum speed (285 kt)
then the Diamond DA-62 (192 kt maximum cruise speed with an absolute maximum of 205 kt) which justifies a slightly higher fuel burn rate. However not 3 times the fuel consumption as indicated by the fuel burn rate. I would like to see them use a pair of 2.5L, 2.7L, or 3.0L 5 cylinder Diesels,  the performance would be closer to that of the single turbine. Especially with use of bigger engines more aggressive props could be fitted to raise the top speed by 80kt.  The service ceiling of the turboprop engine is rated higher. Key word being rated. Who actually knows the maximum altitude the little diesels would continue to give adequate, safe, & reliable performance.
Summary
Turboprop - <50% higher advertised time to overhaul.
- faster and higher rated altitude.
Diesel - <60% less total fuel burned plus the ability to burn actual Diesel and Kerosene not just Jet  fuel as specified by the engine manufacturer.
Diesel engine is cheaper how much though ?  Nobody knows. As the turboprop is very new on the market.
Two engines of any kind are always safer than 1 in the unlikely event that you lose an engine you can still remain airborne for a period of time. Best land quick.
I hope this is answer enough to the question.

The debate about whether twins are better or safer is far from being settled.  That's a different question.

You are still comparing apples and oranges. The Cessna Denali has much more powerful engine (970 kW compared to 2×134 kW) to give it a significantly higher cruise speed. There are turboprops with smaller engines (e.g. Quest Kodiak with 560 kW), but basically, piston engines top at 200 kW and turbines start around 400 kW these days.

Why aren’t diesel piston engines used on aircraft like the Saab 340 or L-410? There are three reasons: weight, weight and weight. Illustrated in this answer, the Rolls Royce Merlin of 2,000 hp weighs 600 kg, the CT-7 of the Saab weighs less than half of that.
At lower power ratings pistons do have a niche, as mentioned here:

So the smaller the engine, the more advantageous are the circumstances for the piston engine: they scale down much more favourably than the gas turbines. But given enough volume, gas turbines are not inherently wasteful with fuel at all.

The niche for aircraft diesels as started by Thielert is in fuel consumption at lower powered engines, compares very favourably with avgas engines. The Saab 340 falls outside the niche.

Consider the efficiency losses of a piston engine when you compare it to a turbine engine that spins on one axis creating uninterrupted power, with intake, compression, combustion and exhaust occurring simultaneously.
The airflow that is effortlessly flowing through the engine is simply injected with fuel which combusts, and expands imparting its energy to turbine blades which turn and drive the prop or reduction gearbox.
No components reversing direction, minimal frictional losses. So simple.
On a piston engine, the intake air has to be delivered to/through the cylinder head at specific points in time by going through intake and exhaust valves which are driven by a valve train, complete with the weight of two camshafts (per head) and the chains that drive them.  All these valves are housed in a complex and heavy cylinder head.
On the current general aviation aircraft gasoline piston engines (which aren't much more advanced than an air-cooled horizontally-opposed flat 4 on a 1960 VW Beetle), the aluminum head will crack if it is shock-cooled during a descent with a low power setting.
However, the Thielert Diesel on a Diamond DA42 TwinStar is water-cooled, which avoids this problem... which reminds me that the cooling system on a jet uses the already abundantly available compressed air created by the engine's compressor section, while a piston engine that uses fluid for cooling needs to enclose the combustion chamber with coolant.
Yet more weight.
Once a piston engine finally meters its air and fuel into the combustion chamber it creates power from gas burning and expanding,
pushing pistons down into a cylinder, which are connected through connecting rods to a crankshaft that must be attached to an engine block strong enough to take all these loads.
Not only are these pistons reversing direction (which makes it possible to "throw a rod"), but the intake, compression, combustion and exhaust cycles occur sequentially, not simultaneously.
This is more wasted efficiency and frictional losses as the piston slides against the cylinder wall (creating no power on 3 of the 4 strokes) as it wears its piston rings away (all of which must be lubricated), as it charges towards the next mandatory engine tear-down and overhaul after only 2,000 hours of service, due to all of this friction wearing piston rings, cylinder walls, camshafts, valves, water pumps, etc...
All These parts that wear out on a piston engine and limit its lifetime, requiring a mandatory overhaul, don't even exist on a turbine engine.
I don't think we even need to compare performance numbers.
If you are going to fly a single-engine piston airplane over an ocean you better make sure your will is prepared because of piston-engine reliability thanks to all these different parts that could fail unexpectedly.
The failure of an oil pump or water pump, alone, will be the end of the engine.
If you are going to fly a single-engine turbine airplane over an ocean it's "just another day at work".
Reliability, maintenance and weight are a few of the reasons that piston-engines haven't been put on airliners or commuter planes for many decades.
It's progress.
Piston engines are early 1900s technology... and, compared to a turbine, they lack power above 5,000' altitude unless you add the weight of yet another costly piece of equipment- a turbocharger.
Nobody wants to fly on DC-6s anymore. Not even a diesel version.  ;)

Two small notes: 1. diesels don't have throttle (“throttle” meters fuel); 2. all diesels are turbocharged these days (the waste-gates needed to prevent overboost at low altitudes are another complication that can ruin the engine if they fail).

@ Jan Thanks, I removed "metered by a throttle and "

One other factor is vibration. Diesels shake a lot, more than gasoline engines due to their high compression ratio, whereas gas turbines, with no reciprocating action, have very little vibration. Aside from making for an uncomfortable ride, that vibration will subject the other components of the aircraft to additional stress, which requires additional reinforcement, which adds additional weight.
Vibration, after all, is a primary cause of metal fatigue.
Plus the nature of component failure in aviation: a vibration related engine mount failure on a big truck isn't a big problem, whereas that same failure on an aircraft can easily end in major disaster.

Modern diesel cars don't "shake a lot" and are nearly indistinguishable from their gasoline counterparts. (Pre-injection helps a lot. Additionally, compression ratios have dropped due to direct injection and turbocharging, a modern diesel is ~16:1 vs 12:1 on gas). Nonetheless, because the proposed aircraft are turboprops, engine vibration is likely to be negligible when compared to propeller-induced vibration.


text:
How slow could the U-2 fly at 50,000 ft?
How slow could the Lockheed U-2 fly at 50,000 ft?
It was surely subsonic as its max speed is 500 mph.

Given that the cruise speed/stall speed get closer the higher you get, "how slow" is probably not much less than it's cruise speed at that altitude, this wikipedia entry about the Coffin Corner says that the stall speed can be as little as 5 knots below cruise speed at altitude for the U2.

You can find the official speed envelope in this copy of the flight manual from '68 that was recently declassified, looks like at FL500 you are right around 105 Kts indicated (convert using figure 2). Also note on this chart it is set up for 1535 gallons of fuel at T.O. the plane had various load configurations (covered in the manual) that would more than likely have an impact on the number,

(figure 1)
If you want to do the correction to TAS you can use this chart also from the manual

(figure 2)
Interestingly the copy from '58 that was also declassified (and hosted on the CIA website) is missing most of these charts although it does not look like they were ever in there.
At higher altitudes the margin was much smaller right around its cruise speed of 373 kts as one of the interesting aspects of the U-2 is that due to its design it was always in and around the Coffin Corner at altitude. According to this former pilot of the U-2

When the littler airplane got up to altitude, the margin between the
  climb speed and the critical Mach number and stall became very, very
  minor, and we were looking at plus or minus two-and-a-half knots at
  one particular point in the climb between the climb schedule and
  critical Mach and stall speed so that’s very very small. When we got
  the bigger airplanes, we got up to plus or minus seven-and-a-half
  knots of margin.

This question may also be of interested to you.
NOTE: The above answer is based on currently declassified information about older variants of the U-2. The plane is still in service and has seen more modern variants which include both aerodynamic and power plant changes to the original designs both factors which could effect the above performance data.

Out of curiosity, do you have any idea why the stall IAS would decrease with altitude?

@Sanchises https://aviation.stackexchange.com/a/25811/1467

@Sanchises: It's a combination of less thrust and a smaller Reynolds number.

It's an interesting tidbit that the aircraft maximum service ceiling was based n this issue, and to reach it required that the pilot engage the autopilot, since the computer was capable of keeping airspeed within a narrower range than a human pilot could.... In fact, Gary Powers was shot down only because his autopilot was inop, and he had to descend to a lower altitude, where the airspeed range was wider, which put him below the max altitude of the SA-2 missile.

The answer to the question is primarily a function of weight and temperature. Asking two of the guys I work with who flew U-2s, they said that there was a big difference flying over Korea vs Vietnam, as the temperatures aloft were more dynamic over Korea.  One simply said that the speed is reached by feel. Stalling a U-2 put one at risk for structural failure in the tail.
So if you work out the indicated temperature you expect, and know what your weight will be, then you know the gap between stall and MMO.
The temperature drives the speed of sound. As the temperature increases, the MMO increases, because the speed of sound increases.
The weight drives the lift needed, and with a lighter aircraft, there is a greater gap between stall and MMO.
So the lowest speed will occur with the lightest aircraft, which is a pretty good spread with the U-2.  Without sensor payloads and with low fuel the stall speed will be the slowest.
Warmer air increases the MMO and widens the coffin corner, and eases the workload.
So what is the number?  Well these guys didn't agree, but they also flew different variant U-2s, and they point out that even from plane to plane they vary, which is why they say the minimum speed is by feel. Also, not many missions are limited to FL500, as at that altitude they have less sensor range. But one, who flew more recently said that he would guess the speed was about 330 to 365 (just below 100 kIAS) for a typical mid-mission weight. He also points out that flying at FL500 is much easier than at higher altitudes, because the spread is pretty wide, and therefore the workload is less, and the probability that the autopilot will work fine is greater.


text:
What is the reason for changing the speed reference (IAS or Mach number) with altitude?
Inspired by that question: How is the airspeed-Mach number transition handled in modern airliners?
When pressure and density decrease IAS also decreases. When temperature and pressure/density decrease speed of sound also decreases. As air pressure, density and temperature all decrease with altitude, if one fly a constant IAS or a constant Mach number while constantly climbing or descending, the aircraft actually constantly accelerates or slows relative to the air mass.
But at lower cruise altitude an aircraft will be piloted to maintain a constant IAS, while at upper cruise altitude a constant Mach will be flown.

Machmeter combined with airspeed indicator, source
What is the reason for changing the speed reference (IAS or Mach number)?

Aircraft are limited by both air speed (VMO, affects loads on the structures) and Mach (MMO, formation of shock waves resulting in buffet).
At low altitudes, the speed of sound is high so an aircraft is most limited by indicated airspeed (IAS). At higher altitudes, the speed of sound is lower so the aircraft will be limited by Mach number. Aircraft typically fly towards the upper limit of their speed, so at some point they will have to switch from remaining under the IAS limit to remaining under the Mach limit.
The chart below shows the changes in different speeds as altitude increases, assuming standard atmosphere.

Trend data source, speeds for 737-800
The aircraft climbs at 250 KIAS from sea level. An acceleration from 250 to 300 KIAS is included at 10,000 feet. You can see that as IAS is held constant, TAS (True Airspeed) and Mach both increase.
The switch from 300 KIAS to Mach 0.76 is done at FL280. You can see the Mach speed is approaching its MMO. From this point, both TAS and IAS decrease. After the tropopause, around 35,000 feet, temperature stops decreasing and TAS remains nearly constant.
At FL400 the aircraft levels off and cruises at Mach 0.765, and a step climb to FL410 is included at this speed to show the trends there.
More detailed charts and an explanation of IAS vs. Mach as it relates to climb and aircraft performance can be found here.

@mins There are two limits: IAS (Vne) and critical Mach number (Mcr). Too much IAS can exceed structural limits. Over critical mach you get undesirable compressability effects (more drag, mach tuck) etc. In warm dense air the critical mach num is higher than the Vne, so you don't need to worry about exceeding it. In cold, thin air the Vne is higher than the critical mach, so you don't have to worry about exceeding that

@mins look up "coffin corner." The "coffin corner" is the angle on the graph where the mach limitation is lower then the IAS limitation

To late to edit, but I think I'm using critical mach wrong. I'm talking about Mmo - the maximum operating mach vs Vmo - maximum operating IAS.

Thank you very much for the good diagram and the clear explanations. The Leeham article is very interesting too.

Strictly-speaking, the speed of sound (its TAS) is actually faster at high altitudes, because sound travels faster in highly-rarefied air; it's just that the TAS-per-unit-IAS increases even faster with altitude than the TAS-per-unit-mach.

@Sean: Speed of sound decreases with altitude, due to temperature decreasing. It's true it increases when density decreases, but this is cancelled by the fact it decreases by the same quantity when pressure decreases.


text:
What are these engine inlet devices used for?
While looking at the Boeing Chinook CH-47 for this question, I noticed the screens in front of the engines, and found similar devices in other photos:

Source: left -- right

What are these devices used for?

A recurrent question here is about protecting turbine engines against bird/FOD ingestion. It seems that a screen can be used, at least at low speed.

It is the case? Is it used for this purpose?

The first photo shows the engine air intake screen. Its basically a screen to prevent FOD ingestion, as you noted. From CH-47 Theory of Operations:

Engine Inlet Screens
An engine inlet screen which maximizes foreign object damage (FOD) is installed on each engine. The reduction in engine power available with screens installed is negligible. The engine inlet screens have bypass panels.  These two panels are on the aft end of each screen. Quick release fasteners attach the panels to the screens. They are removed if the ambient temperature
is 4°C or below and there is visible moisture

I'm pretty sure it is supposed to be minimizes foreign object damage.

It is an All Weather Screen (AWS) intended primarily to prevent ingestion of large items that would cause foreign object damage to the compressor blades.

By Mr.Z-man - Own work, CC BY-SA 3.0, Link
The second photo shows the Engine Air Particle Separator(EAPS) unit, which in this case is the  Pall PUREair System

Pall PUREair system; image from Pall aerospace
The EAPS is there primarily to protect against ingestion of sand and dust, which, according to Why an Engine Air Particle Separator (EAPS)? can cause multiple problems like,

erosion of compressor blades

clogging turbine cooling passages

cause sulphidation, corrosion and overheating

As the article notes,

A CH-47 in the desert with only the AWS could achieve only 12 landings before both engines were changed. With EAPS, it did 13 landings with no change in horsepower. in another desert test CH-47 engines with AWS deteriorated to an unacceptable level in only 30 hours. With EAPS, there was no degradation.

In case of desert environments, EAPS is a must if you want your engine to survive any more than double digit hours. This particular type of EAPS belongs to the Vortex tube separators and uses a vortex generator to separate the sand and dust from air.

Contaminated air entering the ... air cleaner tube assembly is given a swirling motion induced by the vortex generator. This swirling motion causes the heavier dirt particle and water droplets to be thrown radially outward by centrifugal force toward the wall of the tube.

Principle of EAPS operation, image from Pall aerospace.
You can find about other types and their operation in A Comparative Study of Helicopter Engine Particle Separators

I suppose you must be right about the typo for the screens :-) Any idea about what prevents their use in turbofan hot flow and fixed wing turbo/jet/prop?

@mins The operational envelope of helicopters and aircraft are quite different. Simply put, the cost in design, weight, drag, maintenance etc of the screens are not enough to offset their supposed benefits in FOD ingestion in the normal aircraft operating environment. Also, note the point about the reduction in engine power due to them.

@mins I would surmise that the big difference between helicopters and fixed wing in this situation is the speed. You can make a screen that can stand up to bird strikes at helicopter speeds but at fixed wing speed a large bird would penetrate and take the screen with it. They’re also less needed for sand, etc. The debris thrown up by a fixed wing is mostly behind the engine whereas a helicopter is right in the middle of its downwash.

@mins, turboprops designed for use on unpaved runways, e.g. dhc-6, do use “intake deflectors”, which is a simpler kind of EAPS. The simpler version is sufficient because in fixed-wing aircraft the engine is not blowing up debris ahead of itself, so less sand and dust gets ingested.

@mins in a high bypass ratio turbofan, the fan itself acts as a flow separator by centrifuging small pieces of debris away from the engine core and into the bypass duct, where they can't do much damage.

+1 for TomMcW's comment "and take the screen with it". Any "protective" thing you install can easily become a danger to what it was supposed to protect, if not very well engineered and installed. Cost-benefit analysis isn't just dollars-vs-danger, sometimes it's one danger relative to another.


text:
Is it possible to fly a tandem-rotor helicopter after a single rotor failure?
Tandem-rotor helicopters, such as the Boeing CH-46 Sea Knight and CH-47 Chinook, are helicopters with rotors distant by more than 10 m.

Photo by Cpl. Ryan Carpenter on SNAFU
It seems unlikely a single rotor could sustain the force moment created by the failure of the other rotor. On the other hand it seems to me it would be difficult to have one rotor in autorotation and one powered.
If this is possible, how would the pilot fly the helicopter? If it's not possible, why?

If its a structural failure, survival is unlikely. If its a gearbox failure, as the answer says, you can probably autorotate down.

@ROn My Ops O (circa 1982) is one of the few people I know who auto'd a 46 down, and lived, when the aircraft broke in half due to such a failure. The two up front survived, nobody else did.

@KorvinStarmast I'd love to read the accident report on that one. Either way I meant rotor structural failure, not airframe.

@RonBeyer  Possibly the only survivors of an inflight blade loss I was unable to find the mishap my OPSO was in, IIRC it happened during the mid/late 70's).

It's been a few decades since I crewed CH-46s, but the situation you're describing is  called a de-sync and it's a catastrophic failure. The forward and aft rotors are linked by a driveshaft that drives the forward transmission and synchronizes the rotors. In a de-sync the intermeshed rotors will collide and the aircraft will tear itself to pieces.
Also, setting aside for a moment the consequences of a de-sync, the flight controls for both rotors are linked so it would be impossible to flatten the pitch to autorotate just the front rotor head. I suppose it would be mathematically possible to keep the rotors in sync and match the rate of deterioration in the rotor speed, but the odds would be extremely low, and the chances of recovering from any altitude beyond a few feet would be near nil.

@ymb1 It is slightly higher, but not enough to clear each other as the blades flex. The rear rotor is raised to prevent it from striking the fuselage when the cyclic is tiled forward.


text:
How does a runway inspection radar work?
In AC 150/5220-24 FAA mentions radar equipment to detect foreign object debris (FOD) on runways, based on stationary and mobile radars with an astronomical precision, e.g. able to detect an object as small as 3 cm at a distance of 1 km.

Xsight Systems

How do these systems work?
Do they completely replace visual inspections?
Are they common on international airports?

Looking at their patent it appears it uses a multi spectrum system (radar & electro optical) to track all object: birds, aircraft, and FOD.
It uses a radar to detect objects

The potential foreign object source identifier subsystem preferably includes potential foreign object source detectors such as a tower-mounted millimeter wave sensor (MWS) detector

And a specialized light sensor to determine the makeup of the object

FOD identification functionality may be added to the FOD detectors, such as functionality which identifies the material which constitutes the FOD. Remote spectrometry functionality, such as that used in mineral prospecting satellites, may be employed for this purpose.

A neat function too is the ability to determine the where the FOD came from and send pictures of the FOD and the offending source to airport officials

The ascription output may be presented to an airport official in an airport control center on a display console which may show on a first portion  of a screen, an image of the detected FOD and its location, both preferably in a zoomable format, along with a time stamp, and on a second portion of the screen, an image or other representation of a most probable source of the detected FOD, preferably in a zoomable format, along with its time stamp.

It appears it also fuses sensor data with aviation data to augment it's determination

flight database data indicating recent aircraft movements and optical identification data indicating vehicle movements is supplied to data fusion functionality which also receives ASMGCS data and weather data and interfaces

If it works as advertised it not only replaces visual inspection but also provides realtime feedback for bird control and communicating with aircraft falling apart.

KSEA runway 16C/34C has one - https://www.portseattle.org/Newsroom/News-Releases/Pages/default.aspx?year=2015#550

It doesn't have a laser for destroying FOD?  :-(

There are a number of different runway inspection radars on the market - xsight (shown above) Tarsier or FodFinder.
What they have in common is exceptionally small wavelength - a few millimetres - which in radar terms is tiny.   If you want to detect small pieces of debris (nuts, bolts, fragments of metal...), then you need a wavelength about the same size as the objects you are looking for.
However much airports might want to replace visual inspections (nobody wants to close their runway for an inspection, if instead they can bill someone for landing a plane on it) radars have their limits, they sometimes miss things, and don't always work in inclement weather.
A number of major airports have inspection systems, but the majority don't.  Runway inspection is a business of radars, cameras and the Mark I human eyeball.


text:
What is the lightest weight operational jet/turboprop fighter ever deployed in combat?
Since the beginning of WWII there was been a trend towards larger fighter aircraft. In about 1960 the United States continued with larger fighters (F4, F100, F111, F15) but also started a branch of very small light weight fighters (F105, F16, Tucano, etc).
What is the lightest weight jet/turboprop fighter any nation has ever deployed in actual combat?
Epilogue: I learned more from this discussion than I expected. As I recall, at least one South American country used the BD5J but it never saw combat. That's why I included the combat stipulation. - Thanks for the answers...

I would think the Folland Gnat F.1 is hard to beat in this context, with an empty weight of 4,800lbs (2,175 kg) and a loaded weight of 6,250 lb (2,835 kg).
It is credited with a number of victories over the Pakistan Air Force in the Indo-Pakistani War of 1965, principally against the F-86 Sabre.

The A-37 Dragonfly is the smallest production combat aircraft I'm aware of.
Empty weight of  6,211 lb (2,817 kg). Max takeoff weight of 14,000 lb (6,350 kg).

I'm surprised that nobody so far mentioned the Heinkel He-162 A-2. This was powered by a single BMW 003 turbojet and had a maximum take-off mass of 2805 kg, a tad below that of the Folland Gnat. Its empty weight of 1660 kg (3660 lbs) places it clearly below all other contenders. Note that the A-1 variant used heavier armament (2 MK 108 30mm guns), which produced excessive vibrations, so the A-2 version used the lighter MG 151 (20 mm caliber).
And, yes, it was operational, even if only for a few weeks in 1945.

He-162 during post-war evaluation (picture source)
Contrary to contemporary propaganda which claimed that it had been developed in only three months, Heinkel had been working on this configuration for almost a year when the proposal for a cheap, single-engine jet was issued on September 8, 1944. Basically, Heinkel only needed to scale down their design (which was based on a bigger Heinkel engine) and get the prototype into the air. The drooped wingtips were no winglets but served to reduce the dihedral effect and were added when early flight testing revealed dynamic instabilities.

@ymb1: Hehe, yes, good question. For the adherents of the winglet cult it is certainly one, but the intention to add it was not driven by performance, but handling, improvements. This makes it different from a winglet.

Naturally I didn't want to be beaten, but fair play, this may be the winner. However - a note of caution. My initial research indicates the quoted empty weight comes from the unarmed prototype He-162 V1 (which was destroyed in an accident); obviously armament comes at a weight premium. See for instance http://www.world-war-2-planes.com/he-162.html (2180kg empty) and the more authoritative "He 162 Volksjager Units" book (see link below) which quotes a similar weight in its standard (ie combat) configuration.

Link for comment above - https://books.google.co.uk/books?id=ZJUQDQAAQBAJ&pg=PA91&lpg=PA91&dq=he162+weight&source=bl&ots=7cmVrWAwRu&sig=YucPz1TN0Q3imgGlabLw9yP9h-8&hl=en&sa=X&ved=0ahUKEwi4we2izOnYAhVD6KQKHTiUC9gQ6AEIYDAJ#v=onepage&q=he162%20weight&f=false

A bit more - the above book quotes a take off weight for the A1 as 2495 kg. Interestingly in "World War II Fighting Jets" (1994) the author specifically discusses how the He-162 was considerably heavier in operation than the specification, saying the fighter when deployed had a take-off weight of 2805 kgs (6184lbs).
What I would conclude is that coincidentally the empty and loaded  weights of both the operational He-162 and the Gnat were almost identical. In fact this source http://www.ww2warbirds.net/ww2htmls/heinhe162.html gives the weights the same to the kg!

@PartyArk: The A-1 had the heavier MK 108 guns; the A-2 used the lighter MG 151. William Green's "Warplanes of the Third Reich" gives for the A-2 an empty weight of 3,666 lbs and empty equipped weight of 3,875.5 lbs. Take-off weight with 180 Imp. gal. of fuel is given as 6,184 lbs (2805 kg, as you said). The specification called for 2000 kg max; all contestants came in heavier. I simply used the figures in the Wikipedia article in my answer.

Yes, there are lots of sources, and of course with secondary sources you have the danger of blind repetition. For a more definitive answer you'd probably have to track down the reports that were compiled by the countries that captured working examples. Fortunately we have one primary source online - you can read it here, which I think is where Robert Forsyth got his figures. http://www.bredow-web.de/RAF_Museum/Heinkel_He_162/heinkel_he_162.html   That ties in with a completely empty weight of 2050kg as you say, which I suspect is the most credible figure for the combat version.

... and just when I thought the He-162 was the winner, here is better source for the Gnat F.1 giving an empty weight of 4,604 lbs, or 2088kg! http://www.thunder-and-lightnings.co.uk/gnat/history.php  Yet again very similar to the He-162 - I'm calling it a tie :)  (The figure @ymb1 suggested for the Gnat is I think for the better known two-seat trainer version.)

@PartyArk … and the MTOW of the Gnat was 4.1 tons or 9,040 lbs. If we compare apples with apples, this is no tie at all.

The MiG-15 is a good contender having an empty weight of 8,113 lbs and a maximum takeoff weight of 14,458lbs.  Similar too is the Messerschmitt 262 with an empty weight of 8,366lbs and a max takeoff weight of 14,272lbs.

Supersonic aircraft are a bit heavier.  Certainly the MiG-21 is a good contender with an empty weight 12,000 lbs of and a max TO weight of 21,600.
So, too the Lockheed F-104 at 14,000lbs empty and just shy of 30,000lbs at gross takeoff weight.

Perhaps the smallest jet fighter ever conceived, but never put into production is the McDonnell XF-85 Goblin.  Designed as a parasite fighter to be carried aloft by a B-36 bomber, it measures just 14ft in length, with a wingspan of 21ft and a gross takeoff weight of just lbs 5600 lbs!!

F-5 also a contender. 8085 lbs empty weight, 11,477 lbs combat weight, 13,400 gross takeoff weight, although Max takeoff weight was near 20,000.


text:
Why is ARTCC room lighting in the US dim?
I have made several trips to US ARTCC centers and they had very dim lighting. I was told the lighting, carpet and wall colors where all decided on by the (PATCO) union contract with the FAA.
This week I was watching a BBC documentary and the British air route control room (NATS) was brighter than a normal office.
What is the justification between the two lighting preferences.

Dimly lit ARTCC room, even the display and control area use dark trim.

Brightly lit British NATS room, note the bright display and control area.

The ARTCC I visited most (Anchorage Alaska) had one side about like the picture you have a link to, the other side (or row), was so dark I had to wait about 20 seconds to adjust so I didn't hit something.The darkest side included the Air-force liaison officer (he is the guy that knows what the military was planning to do and knew the difference between the SR71 flight plan and a UFO). As the one answer points out, 5 foot-candles is VERY dark. About as dark as a Cessna cockpit with the red light on at night. Even the halls and employee rest area was as dark if not darker then top picture.

In your bottom photo the room looks more modern with very diffuse lighting, much like a regular office. The room in the top photo looks like an older room, possibly designed back when they used crt displays.

That's the FAA for you. The FAA just requested congress to approve an upgrade to the VOR system to replace their 70yr old "tube" VOR electronics. They had wanted to ditch VOR but as a ICAO nation they are committed to another 30yrs so they are hacking sites to the minimum ICAO standards. https://www.faa.gov/air_traffic/flight_info/aeronav/acf/media/Presentations/12-01_Discon-of-VOR-update.pdf

The FAA set out the ARTCC Facility Design Guidelines in this document. In short they are dark to minimize glare.

295. TRACON OPERATIONS ROOM. Lighting intensities in the TRACON operations room are usually kept at a very low ambient level, less
  than 5 foot-candles (fc), in order to prevent glare and reflection
  detracting from the target viewing on the radar displays. The design
  shall consider incandescent lighting with pinhole covers or framing
  capabilities in lieu of fluorescent lights. These incandescent lights
  can be aimed exactly where needed and, unlike fluorescent lights, can
  be dimmed to low light levels.

Its possible the British have no such requirement (under EASA or ICAO) or that the photo was simply over exposed and the room appears bright.

In Britain.. you meant FOGGY right?

Dave, having spent untold hours in the CIC of a USN (Aegis) cruiser, whose core function was to run the air battle (we called the row of radar operators "tracker alley") ... in very dim light ... your first sentence applies to other radar controller applications than ARTCC.  +1

That document (as you quoted) is for a TRACON, a terminal radar approach control, not an ARTCC. In my limited experience US Centers are rather bright compared to TRACONs.

Lower noise and less distractions. People tend to be quieter in a lower lit environment. Better able to focus on what is in front of you when what is around you isn't illuminated. These ergonomics have been lost in newer installations. I worked in the "Dimly lit ARTCC room" above in 2007 and wanted the lights even lower. The newer people wanted the lights higher.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What aircraft broke the most [total] world records?
In the early 1930's aircraft were breaking records each week because the performance needed was quite low. As technology advanced the performance needed to break a record also became higher. Which causes us to ask if it was easier to break records early in the century, or later with the aid of technology.
Howard Hughes built many airplanes specifically for breaking records, yet even today the Russian and American fighter manufactures often strip down their latest fighter prototype and set new world records.

Addendum: There seems to be some confusion over what the question is asking... "most" means "total" as "aeroalias" has supplied below. The basis for the question was a YouTube video that stated the F4 Phantom had broken the most records of any aircraft, but the video was circa 1980 without specifics and it seemed there was a high probability that other aircraft had superseded it. (I would have guessed some aircraft in the 30's, therefore my keen interest in the answer)
At this point "aeroalias" is right on track...

So, what aircraft broke the most (total) world records?

The Wright Flyer broke practically every record pertaining to successful heavier-than-air flight on its first flight. ;-) Speed, altitude, accent rate, thrust, payload... at one time, it lead the pack in all of them. (I say "practically" because you can always add qualifiers, like heaviest payload above 20,000 feet.)

@yshavit - I gave +1 for the humor but realistically and unfortunately for the Wright brothers there were no world records at or soon after their historic flight - Fowl! Answer disqualified!

I'm not exactly sure what type of recored you are looking for (civil, military etc.), but a specially modified Su-27 claimed 41 records for altitude and rate of climb between 1986 and 1990. As globalsecurity.org notes:

Between 1986 and 1990, using a specially configured prototype aeroplane T10-15, which became known as P-42, the design bureau's test pilots established 41 IAF-registered world records of rate-of-climb and flight altitude, some of the records being absolute.

P-42 Streak Flanker; image from su27flankerfamily.wordpress.com
Some of these records are given here,

On October 27 and November 15, OKB test pilot Viktor Pugachev added eight climb-to-altitude time records to the table of world achievements in just in two flights-four absolute records for land based aircraft and four records for aircraft with takeoff weight up to 35,274 lb (16,000 kg). Pugachev climbed to 9,843 ft (3,000 m) in 15.573 seconds and to 19,685 ft (6,000 m) in 37.05 seconds.
On March 10, 1987, and on March 23, 1988, test pilot Nikolai Sadovnikov flew the Sukhoi P-42 to 25,528 ft (9,000 m) in 44 seconds, 39,370 ft (12,000 m) in 55.2 seconds and 49,213 ft (15,000 m) in 70.329 seconds.
On June 10, 1987, Sadovnikov claimed a world record for a N category level flight altitude of 63,435 ft (19,335 m).

In some other publications, a number of 27 records have been reported; in either case, it is quite impressive.

Wonderful!!! I have just watched a YouTube on the SU-27 and record setting flights but it did not give the number of records - great research - I wonder if someone can find the F4 Phantoms achievements to compare? For example the the YouTube video mentions the F4 reaching an astonishing 98,000ft altitude record! https://www.youtube.com/watch?v=aoY1iDnayr4&t=9s

@jwzumwalt Acccording to Boeing, F4 had set a total of 15 World records, so I guess P42 is safe :)

Would be interesting to see some other plane breaking different records, all here seem about climbing.

@h22 this plane on 20.05.1993 set records in maximum weight lifted to 15000m altitude, maximum altitude reached with 1000 kg payload, additionally to some time-to-altitude and time-to-altitude-with-1000kg-payload records, in class C-1i - four world records in the same flight seems quite impressive to me. But most record-breaker aircraft were quite optimized in one aspect - I wouldn't expect to find a plane that held records, for example, in flight distance and top speed simultaneously. Jacks of all trades tend to be masters of none, after all.


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Do you get separate certificates for CFI and CFII?
I passed my CFII checkride the same month my CFI was to expire. So far I have only received a new CFI card in the mail. Shouldn’t I be receiving a card that says CFII on it as well, or should the card I received have read CFII? Are there two separate cards?

If you did it the same month, you probably won't get the CFII card for a while, it takes time to run through the system, especially given COVID shutdowns.

Welcome to aviation.SE! I edited your question to (hopefully) make it a little clearer; if I got something wrong then you can just edit again or roll back. And it may seem like an obvious question, but are you asking about the US? I don't know if other countries use the CFI/CFII terminology too. For questions about qualifications, regulations etc. it's always best to tell which country or regulations you're asking about. Finally, if you're new to the site the [tour] may be helpful.

No.  Under ratings on the backside of the Flight Instructor certificate, it should say:
FLIGHT INSTRUCTOR
AIRPLANE SINGLE ENGINE, INSTRUMENT AIRPLANE.
This is a CFI, CFII Flight Instructor Certificate, assuming you did your training in the United States under the FAA’s system.
You should see the same ratings on your temp certificate as well.

There is no CFI certificate with ASEL on it.  It is classified as ASE or Airplane Single Engine.  You are legal to teach both land and sea planes by passing one practical test provided you have the appropriate class on your pilot certificate.

The answer is found in the FARs.  You will receive a new CFI certificate with the appropriate category and class rating with an instrument rating.  The FAA moniker on the front of the certificate will change from black to blue to indicate a certificate with instrument privileges.

§61.5   Certificates and ratings issued under this part.
(a) The following certificates are issued under this part to an applicant who satisfactorily accomplishes the training and certification requirements for the certificate sought:

(1) Pilot certificates—

(i) Student pilot.
(ii) Sport pilot.
(iii) Recreational pilot.
(iv) Private pilot.
(v) Commercial pilot.
(vi) Airline transport pilot.

(2) Flight instructor certificates.
(3) Ground instructor certificates.

(b) The following ratings are placed on a pilot certificate (other than student pilot) when an applicant satisfactorily accomplishes the training and certification requirements for the rating sought:

(1) Aircraft category ratings—

(i) Airplane.
(ii) Rotorcraft.
(iii) Glider.
(iv) Lighter-than-air.
(v) Powered-lift.
(vi) Powered parachute.
(vii) Weight-shift-control aircraft.

(2) Airplane class ratings—

(i) Single-engine land.
(ii) Multiengine land.
(iii) Single-engine sea.
(iv) Multiengine sea.

(3) Rotorcraft class ratings—

(i) Helicopter.
(ii) Gyroplane.

(4) Lighter-than-air class ratings—

(i) Airship.
(ii) Balloon.

(5) Weight-shift-control aircraft class ratings—

(i) Weight-shift-control aircraft land.
(ii) Weight-shift-control aircraft sea.

(6) Powered parachute class ratings—

(i) Powered parachute land.
(ii) Powered parachute sea.

(7) Aircraft type ratings—

(i) Large aircraft other than lighter-than-air.
(ii) Turbojet-powered airplanes.
(iii) Other aircraft type ratings specified by the Administrator through the aircraft type certification procedures.
(iv) Second-in-command pilot type rating for aircraft that is certificated for operations with a minimum crew of at least two pilots.

(8) Instrument ratings (on private and commercial pilot certificates only)—

(i) Instrument—Airplane.
(ii) Instrument—Helicopter.
(iii) Instrument—Powered-lift.

(c) The following ratings are placed on a flight instructor certificate when an applicant satisfactorily accomplishes the training and certification requirements for the rating sought:

(1) Aircraft category ratings—

(i) Airplane.
(ii) Rotorcraft.
(iii) Glider.
(iv) Powered-lift.

(2) Airplane class ratings—

(i) Single-engine.
(ii) Multiengine.

(3) Rotorcraft class ratings—

(i) Helicopter.
(ii) Gyroplane.

(4) Instrument ratings—

(i) Instrument—Airplane.
(ii) Instrument—Helicopter.
(iii) Instrument—Powered-lift.

(5) Sport pilot rating.

(d) The following ratings are placed on a ground instructor certificate when an applicant satisfactorily accomplishes the training and certification requirements for the rating sought:

(1) Basic.
(2) Advanced.
(3) Instrument.


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What's the point of separating the Alternator and Battery switches?
What's the point of having a different button for the battery? Why does the alternator turn on simultaneously with the battery? I need an easy and short explanation, if possible.

You want a short answer?! You have been on this forum before, right?

I will expand a bit on @Noah Krasser's answer with some situations where you would want one or the other. FWIW under-volt scenarios are typically far less of a problem than over-voltage scenarios.

Bad Voltage Regulator: Alternators vary their voltage output with RPM. To keep your system operating properly a voltage regulator is installed on the alternators direct output. This keeps your systems power supply at a constant voltage, typically at the cost of some power in the lower end. These things can and sometimes do fail, if they fail in such a way that the alternator is at a high RPM and putting out a voltage that could be potentially harmful to your avionics you are going to want the ability to cut it off while still drawing from your battery.
Battery Short: More often than not batteries are stored in the tail section of GA planes or potentially anywhere. If the cable chase leading to the battery develops a short you may want the ability to cut the battery circuit out while still having the ability to power you aircraft from the alternator.
Thermal Runaway: Batteries can runway on you and cause an over-volt issue. Again in flight you are going to want to be able to cut it from the system.

The battery in your link wasn't a thermal runaway, just an overvoltage alternator that likely didn't disconnect from the bus (like #1)

@selectstriker2 updated it with the FAA's AC warning against it instead of that thread. Thanks for the heads up!

Short and concise answer:
Imagine there is a problem with the alternator. You have to shut it down. Now there is only one button to turn off both the alternator and the battery. You just want to turn off the alternator, but you would also lose your radios, the GPS, everything. Is that what you want?

What's the point of turning off the alternator even if it's not working properly (or not working at all) ?  How I can determine that it doesn't function as it should?

@superjet100 Well to prevent it from being further damaged or damaging other components. This might be a bad example but if you find the alternator smoking you would have three choices instead of two: 1) cut the electricity then your engine dies, 2) keep everything running then the alternator catches on fire, then it burns the insulation and shorts the bus, then blows a fuse, then triggers a chain of bad things, 3) cut the alternator and run on battery for a while when you look for a safe solution. Same thing if the battery is faulty.

@superjet100 Also the failure modes of battery and alternator isn't as simple as "fried alternator is no alternator" and sometimes you need a switch to isolate the impact.

@user3528438: Most aviation engines will run just fine without electricity.

@jamesqf Are you indicating gasoline piston engine is minority or aviation gas piston engine run just fine without electricity?

@user3528438 I think what jamesqf meant to say was that most GA engines run just fine without external electricity. They produce their own via the magnetos. They don't use power from the alt to fire the plugs. The only reason a GA engine needs battery power is to crank the starter. Even that isn't necessary if you get out and yank the prop, though.

@user3528438 So the engine won't actually die in your case #1, but you will lose radios, GPS, transponder, lights, electrically-driven instruments, and other things that are generally considered to be nice to have while flying.

An example of a faulty alternator that was eluded to is an alternator that is pumping too much current into battery and electrical system (overvoltage). In this case, you may want to turn off the alternator before it damages the electronics and/or battery, but you still want to be able to use electrical equipment in the airplane, which will run for a certain period of time directly off of the battery.

@reirab: Exactly - and if I find my starter isn't working when I'm out at a nice, remote dirt strip, that's exactly what I do, unless I really want a long, long walk or bike ride.  (One instance I particularly remember is located at about 40.98852, -118.9621 :-))

@user3528438 In fact most GA engines have two magnetos, and two spark plugs per cylinder, just in case a magneto fails. Checking they both work independently is part of the standard pre-takeoff checks. You can't fly far with no engine, but you can get along just fine in an emergency situation with no GPS, radio, etc - so long as you can still remember how to read a paper map, and you actually have one in the cockpit...

In addition to the other answers, most modern alternators have stator and rotor coils. One generates the electricity, the other consumes current to generate a magnetic field. When driven by the engine, the current consumed is a small fraction of the current generated.
However, when you first power up your machine you really do not want the batteries delivering current to the alternator till after the engine is started and running or you will reduce the amount of battery power available and eventually drain the battery.
As such, having the ability to completely disconnect the alternator is a good thing.


text:
Why don't competition gliders use riblet skin surface to reduce drag?
Price is not a problem in competition gliders, also no problem with in maintenance surfaces like at airliners.
It is very easy to wash glider before every race.
A riblet surface is proven (PDF) to have lower frictional drag.
Why do full scale  gliders not use riblet technology or even matte the surface with 1000 grit sandpaper as the windsurfing industry has for many years?
Has anyone tried to sand surface and try compete against regular glossy gliders?
Birds wings are rough too, do they also benefit from this phenomenon?

There are many variables in the aerodynamics which make a surface appropriate for one use but not another. Controlled turbulent boundary layer(rough/textured) surfaces may have lower average drag over a wide range of conditions, but laminar boundary layer (clean glossy) surfaces can have much lower drag in a narrow set of conditions. The airfoil profile also matters greatly to the effects of surface finish. Reynolds number is a large variable too, birds have an extremely low Reynolds number and human carrying gliders have a medium Reynolds number. Reynolds number drastically effects the behavior of the boundary layer.
There is also consideration for strength and ease of manufacture and maintenance. If it harms the strength to weight ratio, if it creates reliability and inspection problems in manufacture, if it is not readily maintainable.
The assumption that cost is not a factor is also completely erroneous. All competitors have limited budgets, it may be a large limit but it is still a limit.


text:
What is this low wing propeller plane flying near Denver?
I have been trying to identify this plane seen near Denver on August 21, 2021. I have searched online but can’t find any matches. Plane (with smoke) was part of a flyover for a memorial service at Ft. Logan National Cemetery south of Denver around 10am on August 21.

Plane is over Ft. Logan National Cemetery south of Denver around 10am on August 21, 2021. Smoke is part of a flyover for someone else’s service, though I was not able to discover details. My first thought was a Texan due to the wing shape but rudder and canopy didn’t seem to match. An earlier variant is an idea to pursue. Thanks. Perhaps a modification to try to look like a P47?

The radial engine and straight trailing edge point to a T-6, but canopy and empennage seem different.

Not  much  of a photo, but Curtis P-36 perhaps?

Do you have a time and a more accurate location?

My first thought was P-36 as well, but they are quite rare.

People keep claiming that it’s a T-6, but the wingtips don’t seem right. They’re a lot more rounded than any T-6 I’ve seen.

The plane in the photo is clearly some variant of the North American T-6   / SNJ "Texan", or its precursors.  Identifying features visible in the photo include the wing with straight unswept trailing edges and swept leading edges, the shape of the horizontal tail, and the distinctive fairings for the retracted wheels, under the fuselage just forward of the leading edge of the wing.
The North American SNJ-2, a Texan variant, has a rudder with a wide chord (width) near the top and a curved trailing edge, like the aircraft in the question. The photo below came from  this website.
The aircraft in the original question appears to be an example of this type or another closely-related variant.  (Earlier predecessors to the Texan, while much less common today than the SNJ-2, had similar rudders.)  On the aircraft in the original question, in addition to the (obvious) olive-drab paint scheme with early WW2-era US military insignia, note the prop spinner, painted light blue in front, and the three-bladed prop.  The engine cowling also appears to have an unusually long chord (width).  These features are undoubtedly post-production modifications, and it's unlikely that an early, rare, predecessor to the Texan would have received these modifications, bolstering the case for the SNJ-2 or another very closely-related variant. And the search goes on--
(Link to Wikipedia article on "Texan" variants and precursors)

Sooner or later someone will nail down the exact individual airplane--

I was ruling out the T6 because of the curved trailing edge of the rudder, but this has got to be it.

I suppose I ought to delete the actual photo for copyright reasons and just give the link, as per another recent answer?

"early, rare" should read perhaps "earlier, even rarer" -- w/ only 61 built, the SNJ-2 is surely "rare" as well --

The photo credit is there, why delete?  I can’t imagine  Jetphotos complaining about free publicity, and you aren’t benefiting commercially from the photo.  (Well, this is aviation, maybe the FAA considers reputation points to be “compensation”?!)

This aircraft is definitely a SNJ-2. Only 61 SNJ-2s were built. The insignia and tail markings are typical for 1942.  The dark painted color is typical of the SNJ-2 in Navy service.
These aircraft were built by North American Aviation at the Inglewood California plant prior to 1941, and later as SNJ-3/AT-6s at the NAA Dallas Texas plant beginning in March 1941. An almost exact view at the same angle of one of the Geico SNJ-2 planes trailing smoke may be seen here. The photograph is not reproduced to protect copyright.  North American Aviation, by the way, was responsible for the design, lofting, and prototype assembly of the P-51, all in approximately 90 days. War-time production of the P-51 was at the Dallas plant.
The SNJ-2 is the predecessor of the SNJ-3/AT-6A and succeeding variants that were used as pilot-training aircraft throughout the Second War. The AT-6, in particular, and its successive variants, remained in service as a pilot training aircraft well past 1960.
As just a side note, many people incidentally know the answers to these questions and, just to be certain, spend hours doing on-line research to verify, validate, and firm-up an answer given here, on SE-Aviation.  Some of this information is not easily derived, is sometimes traceable through anecdotal information or memory, or is discovered hidden in obscure ancillary sites and well documented blogs. Even though pinning down the particulars and validating an answer is hard work, there is still a chance that errors may be present in the answer given. The pieces of the puzzle are many and don't always fit together well.  Most of the experts regarding the history and use of these planes are now gone.  Not being an expert, this was nevertheless, a very interesting question to answer. Thanks for the opportunity, and the chance at discovery.

Just out of curiosity, how did you absolutely rule out, say, the SNJ-1, or other even earlier precursors?  I suppose the likely answer is "because none are flyable" -- but it might be worth incorporating that info into answer.

( And, obviously, if you've tracked down the exact individual airplane, do incorporate that as well! )

I can't identify the exact tail number, but I believe it's a T-6 or variant. This facebook post from Rocky Mountain Memorial Airport (Broomfield) two days later says "Wondering about that low rumbling and shiny planes you perhaps just saw overhead?" and says it was part of the commerative air force "air power squadron"; they have at least seven planes in the SNJ/T-6 family. They didn't promise which T-6 they were bringing, but they were offering rides in it August 25-29.
So, there was a T-6 in Denver from the 23rd to 29th. It's safe to say it might have been spotted two days before that.
Other leads- the Texas Raiders were flying over Denver on September 3 2021, Exploration of Flight museum has a flyable SNJ (Navy T-6). Wings Over The Rockies owns this T-6. Exploration of Flight was closed on August 21 2021 for a private event.

I wouldn't call it shiny either.

The rudder shape is wrong.  If it's a T6, it's modified.

I think it is N9790Z which is a North American AT-6A belonging to the Houston Wing of the Commemorative Airforce.
Commemorative Airforce AT-6A

The OP photo doesn't seem to have red fuselage stripes, red wingtips, a straight rudder trailing edge or air scoops by the engine, and seems to be darker than this.

that plane has a radial engine, retractable landing gear, a wing with a straight trailing edge and a tapered leading edge, pseudo-military markings, and is being used for skywriting. This means it is probably a T-6 "Texan".

@quietflyer Why not a T6? Wing, radial engine, rudder shape all look right.

disagree. I think it's just an optical illusion. The cloverleaf horizontal stab, which is forward of the rudder, also identifies it.

This is essentially a correct answer.  The aircraft is a variant predecessor to the T-6 likely  built around 1940 by North American Aviation.

@ThomasPerry I think you are right, the North American BC-1 has the rounded trailing edge of the rudder the AT-6 is missing. My money is on a BC-1.

@PeterKämpf Thanks for the note, Peter. Investigating comments to my posted answer; evidently the type designation and mfg date have been questioned. Trying to sort this out... There may have been modifications to parts and engineering before the airframe was modified or redesignated. Some of this may be unknown, except by incidental information. Almost all SNJ-1s were crashed/ground-looped. Unknown number repaired.


text:
Are accident reviews comprehensive enough?
There is lots of speculation on accident causes. How complete is the FAA / NTSB analysis of accidents in terms of detailed responses from the manufacturer and expected corrective actions that will be taken to lower the chances for a repeat?
Are actions addressed in terms of management, design, quality control and customer actions that contribute to the risk? Is NTSB and manufacturer analysis critiqued by independent reviewers for completeness?

What makes you think this sort of thing doesn't happen?  (FWIW, it is the NTSB that investigates accidents.)

Seems like "comprehensive enough" is only answerable with opinions (although the existing answer makes the strong case that for any reasonable opinion, the answer would be "yes").

Is it an chicken-egg problem? FAA publishes accident reviews to make aware other agents about dangers and corrective actions, so it cannot include responses on that. On other reports (annual reports and specific report published less often) you may get data about effect of the published report.

I wonder what aircraft accident investigation reports you've read that do not include all these things.

Yes, they are very comprehensive.
As officially stated, reports do provide details about the accident, analysis of the factual data, conclusions and the probable cause of the accident, and the related safety recommendations.
Commercial aircraft accident investigations are significantly more thorough than they are for road vehicles, buildings, industrial plants, consumer products, and just about anything else. Their conclusions are also acted upon swiftly and firmly, which is how an inherently challenging activity was turned into the safest mode of transportation on a per-mile basis.
For class 1 investigations (airliner accidents that make the news are usually class 1), the report is typically between 200 and 400 pages, including diagrams, illustrations, context, and plain language explanations. In most countries, these reports are very well-written, following a similar structure. Most of these reports are easily readable by the general public, requiring only a modicum of knowledge of basic engineering or aeronautical terms.
Here are some select reports:

Famous TWA 800 - 747 midair explosion
Ethiopian 737 MAX 8 - note it's foreign, but follows a similar template
JAL 123 - deadliest single aircraft crash, 747, Japan (hard to read, see below for FAA version)
USA 1549 - A320 on the Hudson, basis for the Sully movie
Qantas Flight 32 - engine failure, A380, very well-illustrated

Note that they are produced by the NTSB, or by other nations' safety agencies - typically, the operating country makes the final report. However, different national agencies, including always that where the accident occurred, that where the aircraft was designed or built, the airline's flag country, and sometimes others, collaborate on producing it.
The process is simpler for private flights. If it's a Piper crash-landing, or a seaplane clipping a motorboat, that's a class 4; they make a prelim, take a month to do the paperwork, and release a 4 to 10 page long report. If it's an airliner accident, the investigation takes as long as it takes to find the proximate cause and propose a solution. If they have to reassemble the airplane from small pieces, they do it; the longest investigation took almost 5 years (report on USA 427). It doesn't stop until it's done.
The acceptance rate and the speed of implementations for air safety recommendations are much higher than in other industries. About 85% of class 1 investigations result in changes in practices or retrofits within a year. Most marine, rail, road and building safety improvements only apply to new designs, with the exception of specific critical faults.
All information of interest to the general public is included in the Final Report. I strongly recommend reading at least TWA and Qantas reports if you want to get a sense of how they're done and what they contain.
If you want even more information, look through the dockets, which at least in the US are also made public. In a class 1 investigation, it can total thousands of pages, as for the TWA 800 investigation. That's literally everything, including investigation reports into rejected versions - e.g. a 24-page report on the missile impact hypothesis. Some of these papers require engineering knowledge to understand.
The FAA also maintain a page with even shorter summaries for most influential accidents. It's particularly useful for accidents from non-English-speaking countries, where different writing culture can make official translations hard to digest. FAA's JAL 123 summary is much easier to read than the Japanese version above.

Therac, great call. A long time ago, I took an Accident Investigation & Analysis course taught by one of the TWA 800 investigators. It was one of the best classes I’ve taken.

I'd add this link to your answer which is an FAA site explaining in a simple but thorough way the "most significant" incidents happened in the past. Even the JAL123 is explained better than the official report

@sophit Thanks, added. I think it's indeed a better starting point for someone uninvolved with aviation; somewhere between the wiki and the official report.

To focus on the safety recommendations part of your question:
As mentioned in Therac's answer, the FAA is not the primary agency investigating aviation accidents. The NTSB is the US agency responsible for investigations. This provides independence between the investigation and the regulation, but it also means they have no authority to change anything and can only issue recommendations.
Especially for more serious incidents, the thorough NTSB reports will often contain a section with recommendations to the FAA or other relevant bodies. The scope of the recommendations is not limited to just the specific incident being investigated but may include additional findings that happen to come up in the course of the investigation. These recommendations are separately tracked and the NTSB notes whether there is an acceptable response that addresses the concern. In 2022, the NTSB closed 269 previous recommendations as having acceptable responses, and 34 as having unacceptable responses.
Here is a chart of the recipients of all recommendations since the NTSB was created in 1967. This includes all modes of transportation they investigate for; aviation comprises 38% of these recommendations.

Source: NTSB